This invention relates generally to generation of high power laser beams using arrays of optical fibers and, more particularly, to techniques for more efficiently combining multiple beams emanating from an array of fiber elements. Fiber arrays have been employed to combine the relatively small powers output from the individual fibers, to generate a composite beam of much higher power. Such arrays are well known to have a number of directed energy applications in both military and commercial contexts.
When multiple sub-beams or “beamlets” from separate fibers in an array are combined to form a single composite beam, the energy distribution across the composite beam is an important factor that provides a measure of the efficiency of the beam. In the near field, close to the ends of the radiation emitting fibers, the individual beamlets retain their character as separate sources of radiation, and there will necessarily be distinct spaces between the beamlets. At distances farther from the array of fibers, the beamlets have diffracted and diverged to an extent that they overlap and merge. In most applications of high power lasers, the nature of the resulting composite beam at distances comparable to the “far field” is what is of greatest interest. The far field is generally defined as being at or beyond the distance at which the angular field distribution is independent of the distance from the radiation source. The far field distance is proportional to the square of the diameter of the source, and is inversely proportional to the source wavelength.
Unfortunately, even if the beamlets from individual fibers are collimated the resulting far field energy distribution pattern is characterized by a central lobe and a number of smaller side lobes surrounding the central lobe. The far field encircled energy may be defined as the percentage of total energy that falls within a prescribed circle in the far field. Typically the circle is defined to encompass the energy in the central lobe and to exclude the side lobes. The encircled far field energy, when expressed as a percentage of the total energy emitted by the fiber array, provides a measure of the efficiency of the optical system that produces the composite beam, the assumption being that energy falling outside the prescribed circle is wasted.
The far field encircled energy depends on several physical and optical parameters of the array, including fiber array pattern and specifically its “fill” factor, the emission profile of each fiber, and the overall emission profile of the composite beam. Prior to this invention, attempts to scale to higher power outputs by combining beamlets from multiple fibers have resulted in a significant fraction of the output power falling outside the central lobe, even when collimating lenses are used. This phenomenon is due in part to near field intensity modulation across the array. A far field encircled energy of approximately 60% was considered normal prior to the invention. Increasing the encircled energy provides increased energy at the target of the composite beam, minimization of stray light, in the side lobes, and increased overall efficiency. Increased overall efficiency translates, of course, into the ability to produce a prescribed energy beam with a system that uses less power and has lower weight, or permits the generation of higher energy beams without increasing the power and weight of the system.
It will be appreciated that there is a significant need for a technique for optimizing energy distribution and, more specifically, increasing far field encircled energy, in a beam that combines beamlets emitted from an array of fibers, thereby increasing the overall efficiency of a high power laser source. The present invention satisfies this need.